Poor adherence with medication therapy has been estimated to be responsible for more than 5 percent of all hospital admissions, with an associated direct cost exceeding $8 billion. This is a particular problem with drugs that have a narrow therapeutic index. Warfarin is such a drug. If used correctly, warfarin is highly efficacious in preventing thromboembolism (TE) in patients with conditions that put them at risk. However, errors in its use can lead to life-threatening thromboembolic and bleeding complications. In addition, errors can lead to increased medical costs, reduced quality of life, patient dissatisfaction, and discontinuation of a highly efficacious therapy. Because of the numerous conditions that put patients at risk, warfarin is the tenth most commonly prescribed medication in the United States. As a result of this widespread use and the drug?s narrow therapeutic index, warfarin errors are extremely common. Despite our medical knowledge and attendant attempts to control many factors that can affect anticoagulation levels, proper anticoagulation is extremely difficult to maintain. Even in optimal clinical settings, about one-half of all patient time is spent either under-or over-anticoagulated. In our hospital, complications from warfarin are the most commonly reported adverse drug reaction and warfarin is the leading cause of admissions for serious adverse drug events. Even "near misses" can lead to increased medical costs and discontinuation of an otherwise extremely effective therapy. Poor patient adherence with warfarin therapy is often cited as one of the most common errors with the drug. However, the problem with adherence has not been addressed, and no study has rigorously attempted to identify predictors of poor adherence with warfarin. Therefore, the purpose of this study is to determine the human and system factors (both patient and medical practice specific) that lead to errors with warfarin, specifically focusing on adherence. The aims are to perform a prospective cohort study of anticoagulation patients to determine the specific clinical, demographic, health care structural, pill-taking practices, and psychosocial factors associated with poor adherence, and to develop a predictive index that can identify patients at risk for medication errors. The hypothesis is that specific clinical and demographic characteristics, psychosocial factors (e.g., poor cognition, passive coping strategies) and variables describing medical practice (e.g., time providers spend with patients, primary care physician support) and medication-taking practices (e.g., reminder systems) will predict poor adherence with warfarin. The ultimate goal of this research is to provide a more complete understanding of the epidemiology of patients' errors.